runtime: deflake TestCgoPprofThread
runtime: deflake TestCgoPprofThread

In TestCgoPprofThread, the (fake) cgo traceback function pretends
all C CPU samples are in cpuHogThread. But if a profiling signal
lands in C code but outside of that thread, e.g. before/when the
thread is created, we will get a sample which looks like Go calls
into cpuHogThread. This CL makes the cgo traceback function only
return cpuHogThread PCs when a signal lands on that thread.

May fix #52726.

Change-Id: I21c40f974d1882508626faf3ac45e8347fec31c4
Reviewed-on: https://go-review.googlesource.com/c/go/+/406934
Run-TryBot: Cherry Mui <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@google.com>

nawawi/go,golang/go,golang/go,nawawi/go,golang/go,nawawi/go,nawawi/go,golang/go,nawawi/go,nawawi/go,golang/go,nawawi/go,golang/go,nawawi/go,golang/go,golang/go